This disclosure relates to the delashing of worm gear assemblies, and, more particularly, to the delashing of a worm/worm gear assembly through the use of double flank contact maintained between a worm gear and a worm.
Gear assemblies typically comprise a drivable gear (e.g., a worm gear) engaged by a pinion (e.g., a worm). In order to permit relative motion between the drivable gear and the pinion, a clearance should be maintained between the teeth of the drivable gear and the teeth of the pinion when the two gears are engaged. This clearance is known as backlash. In a gear assembly with no backlash, the meshing of the teeth between the gears will be so tight that, absent any deflection of the teeth, the gears will bind and cause the gear assembly to jam. In a gear system with any degree of backlash, however, an audible noise is produced by the movement of the teeth of one gear through applicable backlash and into contact with the teeth of the other gear.
In a worm/worm gear assembly, a worm gear is engaged and driven by a worm. Both worm gears and worms are typically fabricated from metal and are configured such that the teeth of the worm are aligned with the spaces defined by the teeth of the worm gear. The teeth of the worm are usually aligned with spaces defined by the teeth of the worm gear such that clearance exists in the meshing of the teeth of each gear. Furthermore, because of the inflexibility of the metallic teeth, the planar surfaces of the flanks, and the need to maintain the clearance, one flank surface of each tooth of the pinion engages one flank surface of each tooth of the driven gear to result in single flank contact. Systems using single flank contact typically employ a contact area uniformly distributed across the surfaces of the engaged flanks. The uniformly distributed contact area, in conjunction with the clearance between the engaged teeth, is a factor that contributes to the audible noise produced during the driving of the worm gear by the worm, particularly during periods when the rotational direction of the worm is reversed to reverse the direction of rotation of the worm gear. Such noise, although not indicative of a defect in the worm/worm gear assembly, generally proves to be undesirable, especially when the worm/worm gear assembly is located within the passenger compartment of a motor vehicle.
The reduction of backlash and the minimization of noise have been achieved in the related art through the use of split gears, which typically comprise two gear halves mounted side-by-side and wherein each gear half includes a plurality of teeth. A spring is positioned between each gear half to bias each gear half in opposing rotational directions. The opposing rotational directions cause flank surface contact to be maintained between a tooth on one of the gear halves and a tooth of a gear with which the split gear is in contact while simultaneously causing flank surface contact to be maintained between the aligned tooth on the other of the gear halves and an opposing flank surface of the tooth of the gear with which the split gear is in contact. Such a structure allows for double flank surface contact to be maintained between successive teeth in a gear set, which reduces or eliminates backlash in the gear set, thereby minimizing noise. However, because of the tension on the spring, both the contact force and the size of the contact area maintained by the teeth of the split gear on the successive teeth of the gear with which the split gear is in contact remain constant regardless of the load imposed on the gear set. Constant force and contact area size cause the contact pressure to vary, which thereby causes constant frictional forces to be experienced by the gear flank surfaces as the load varies. Such constant frictional forces in turn cause a constant degradation of performance within the gear set.
A worm/worm gear assembly comprises a metal worm and a worm gear fabricated from a resilient material. The worm and the worm gear are maintained in compressive engagement such that double flank contact is maintained therebetween in low- and no-load conditions. One use of such an assembly is to provide torque assist at the steering column of a steering assist system of a motor vehicle. The torque assist is effectuated using a drive motor and a hand steering device maintained in mechanical communication with the worm/worm gear assembly. The hand steering device, through which operator inputs are received by the steering assist system, is typically a hand steering wheel. The operator inputs are sensed through torque and position sensors in electronic communication with the steering assist system. The proper amount of torque assist is then applied to the worm/worm gear assembly through the drive motor to assist in the rotation of the steering column.
The worm/worm gear assembly comprises a worm having teeth defined by at least one thread disposed thereon and a worm gear having flexible and uninterrupted teeth protruding from a surface thereof. The worm and the worm gear are interengaged and in a compressive relationship with each other such that opposing flanks of the teeth of each interengage. Opposing flanks of each tooth are typically arcuately formed. Causing the opposing flanks of a tooth of the worm to engage the facing flanks of two successive teeth of the worm gear results in xe2x80x9cdouble flank contactxe2x80x9d. At low- or no-load conditions, the contact between the flank surfaces is maintained at the opposing outer edges of the teeth of the worm gear. At moderate-load conditions, the contact between the flank surfaces extends from the opposing outer edges of the teeth toward the center of the teeth of the worm gear. At least one of the worm and the worm gear are fabricated from a resilient material such as a polyamide in order to allow for flexibility between the teeth of the worm and the worm gear during double flank contact. The flank surfaces of each of the teeth of the worm and the worm gear are contiguous so as to provide an uninterrupted boundary for a lubricant disposed on the assembly.
A method for delashing a gear system comprises pressing the worm into mechanical communication with the worm gear to result in double flank contact. The double flank contact is maintained at the opposing edges of each of the successive worm gear teeth and consequently the worm teeth during low- or no-load conditions and extends toward the center of each of the successive worm gear teeth and consequently the worm teeth during moderate-load conditions. The loading of the worm gear to effectuate the compressive engagement typically includes matching the worm with the worm gear to attain the most efficient combination for the optimum performance of the worm/worm gear assembly.
A method of transmitting the detections of variations in a road surface over which a motor vehicle having the worm/worm gear assembly incorporated therein includes disposing the worm in a flexible relationship with the worm gear, mounting the worm/worm gear assembly into a steering assist system, maneuvering the motor vehicle over the road surface, and sensing variations in the road surface through a steering device disposed in mechanical communication with the steering assist system.